There are 12 notes and what seems an endless combination of those. However, certain combinations will sound better than others and this reduces the number of possibilities significantly.

It's not surprising then to see that some of these music pieces look alike.

But there's another effect that you touch briefly, borrowing music from other artists unconsciously. It's so synchronic that you touched the familiarity of songs at the same time I was writing about this but from this subconscious point of view. Maybe you'd be interested in checking it out. It's called the

The Cryptomnesia Effect:

https://medium.com/p/12a5e567605f

Anyway, thanks for bringing awareness about this topic.

Cheers!